There are known a variety of devices which monitor input voltage and select an appropriate transformer primary winding tap to attempt to maintain a constant output voltage. These devices are designed to provide a standard nominal level of electrical power so that electrical equipment may be used anywhere, regardless of fluctuations in the power supplied from a local power grid or the like. As should be recognized, the power grids of many areas may not supply constant regulated power, severely impacting proper operation of electrical equipment. Known devices automatically sense the magnitude of the input voltage supplied from the power grid, and attempt to supply a nominal output voltage, commonly being 100, 120, 220 or 240 VAC. A transformer is used in conjunction with an appropriate transformer tap to provide the nominal output voltage. To maintain the output voltage over a wide range of inputs, the transformer has a plurality of taps, and mechanical relays are commonly used to couple the input voltage to the transformer taps to allow the output voltage from the transformer to be decreased (stepped down) or increased (stepped up) to maintain the desired nominal output.
Although providing a nominal output voltage from a fluctuating input, such devices have been found to be deficient in several respects. The devices normally use discrete components, including the use of relay contacts to reconfigure transformer primary windings. Upon detecting a change in input voltage, such devices automatically change taps, requiring a reconfiguration of the state of the relays to produce the desired nominal output voltage. To maintain the desired output voltage, reconfiguring relay taps was normally performed after current to the transformer as well as the mechanical relay coils was reduced to zero, to avoid breaking the relay contacts under current. If current were still applied to the transformer and mechanical relay coils, breaking the contacts under current will generally result in arcing of current from the transformer to the contacts, resulting in deterioration of the contacts. Changing taps under zero current avoids such deterioration, but requires a finite amount of time to reduce the current to zero and effect a tap change. Further time is necessary to let the tap settle down before current is reapplied to the transformer. As an example, the time to turn off current, change the tap selection and reapply current to the transformer may require 8 to 10 seconds where power is not being supplied through the device and to downstream electrical equipment.
In many situations, such devices are used with UPS devices, which allow power to be continuously supplied, even during transformer tap reconfiguration, by means of the battery power supply provided in the UPS. Although continuous power is supplied, relying upon the battery power supply of the UPS to supply continuous power results in significantly reduced battery life in the UPS. Alternatively, if such a device is not used with a UPS, tap selection reconfiguration results in a momentary power outage to the supplied electrical equipment. In known devices, even if used with a UPS, an auxiliary output is normally provided to couple power to non-critical electrical equipment such as laser printers. For a laser printer or other equipment, the momentary power outage produced upon reconfiguring tap selection of the transformer causes the laser printer to reset repeatedly as power is cut off and reapplied. Such devices were thus usable only as an autoconfigurable transformer to provide a desired nominal output voltage, but were not usable as a power conditioner for electrical equipment due to such problems.
Another deficiency in known devices is found in that their response to varying input voltage conditions is based upon the discrete components used therein. The various voltage trip points or thresholds at which transformer tap selections are reconfigured are subject to the selection and tolerances of the components themselves, and simply provide preset voltage trip points for reconfiguring the transformer taps. The devices thus provided a go/no go decision for tap selection based upon a varying input voltage. The go/no go decision has some delay time, based upon the discrete components used, which causes problems similar to that described above. This does not account for certain line conditions which can cause intermittent dips or surges in the input voltage, such as for example when a laser printer is initially turned on and heats up. In this example, the laser printer places high initial current demands on the power supply, but such demand only last momentarily. The fluctuations in the input voltage caused by such occurrences would cause known devices to respond by changing the transformer taps to compensate for the voltage drop. This can result in the device attempting to reconfigure the transformer tap selection to account for the momentary fluctuation in input voltage, and shortly thereafter to flip/flop back to the original tap selection as the device tries to catch up with voltage changes. The use of discrete components also makes it difficult to adapt decisions in reconfiguring tap selections to a particular environment or application. For example, if the device is turned on with the transformer set with a low voltage tap, such as a 100 volts, but the input voltage was 140 volts, the device must change to a higher tap selection, such as the 220 volt tap as an example. Similarly, if a high voltage tap such as 220 volts is selected at start up, line failure or loss of a current phase would cause a voltage drop requiring change to a lower tap selection, even though such conditions were momentary. The discrete tap selections provided by the devices thus may not account for all conditions experienced in an application or environment in which such an apparatus would be desirable.
Known devices are also deficient because they don't account for initial transformer inrush currents when electrical loads are initially energized. Transformer inrush current is the result of core saturation and it can reach a magnitude of ten times the nominal current rating of any electrical equipment acting as a load. Maximum inrush current flows during the first half cycle of the input voltage and then decays within several cycles of the final steady state value. The severity of the inrush is dependent on the relative timing of the removal of line voltage and its reapplication to the transformer. Worst case timing for inrush current is realized when line power is removed just after the transformer magnetizing current cycle has peaked and before it has begun to flow in the opposite direction--leaving the transformer core magnetized. If power is reapplied such that the first half cycle of magnetizing current flows through the transformer in the same direction as when it was last removed, the already magnetized core will be driven into saturation. At saturation, transformer inductance falls to a fraction of its nominal value leaving the relatively low resistance of the transformer winding and the mains or input power line resistance as the only limit to inrush current.
In known devices utilizing a transformer and having a manually operated contactor, the timing of power removal and reapplication will be at random times so that best or worst case scenarios for inrush current are not always realized. However, inrush currents high enough to false trip protectors regularly occur in installations, especially where the transformer has a VA rating near the capacity of the circuit.